Not showing a lot of promise during his martial arts training, a 16-year-old Gensai was appointed to Kumamoto Castle to basically be its janitor. Then one year, he accompanied his lord to Edo (modern-day Tokyo) … just as Matthew Perry arrived in Japan. Seeing the country eventually surrender to a foreign power created a strong sense of "Oh, HELL no" inside Gensai, and he decided to do brutal murder about it.
See, it turned out that the reason Gensai didn't do that great during training when he was young was that he thought fighting with bamboo practice swords was a waste of time. Later, when everyone stopped paying attention to him, he practiced with a real katana in between his sweeping-up duties, eventually developing a lightning-fast style that took advantage of his small size. He was reportedly able to use just one hand to pull out his sword from a low crouching position and cut a man in half from balls to brains. He quickly became one of the most fearsome assassins fighting for the royalist side, earning himself the nickname of "The Viper" … you know, to go with his other badass nickname of one of the four "great manslayers."
via Wiki Commons
A lot of assassinations (often carried out in broad daylight) were attributed to Gensai, but only one has been confirmed. Sadly, it's not the one where, according to legend, he was drinking with his fellow royalists when someone started talking about a particular supporter of the shogunate who could particularly eat a mountain of dog doo doo. Upon hearing this, Gensai reportedly went out and later returned to the party with the man's head dripping blood all over the floor. There's no proof this really happened, but it'd make sense because while everyone described the man as a calm and kindhearted friend who loved his family, Gensai was also known for being especially vicious and brutal during combat.